The control of infection by plant pathogens, which can inhibit production of fruits, seeds, foliage and flowers and cause reductions in the quality and quantity of the harvested crops, is of significant economic importance. Pathogens annually cause billions of dollars in damage to crops worldwide (Baker et al. 1997, Science 276:726-733). Consequently, an increasing amount of research has been dedicated to developing novel methods for controlling plant diseases. Such studies have centered on the plant's innate ability to resist pathogen invasion in an effort to buttress the plant's own defenses to counter pathogen attacks (Staskawicz et al. 1995, Science 268:661-667; Baker et al. supra).
Although most crops are treated with agricultural anti-fungal, anti-bacterial agents and/or pesticidal agents, damage from pathogenic infection still results in revenue losses to the agricultural industry on a regular basis. Furthermore, many of the agents used to control such infection or infestation cause adverse side effects to the plant and/or to the environment. Plants with enhanced resistance to infection by pathogens would decrease or eliminate the need for application of chemical anti-fungal, anti-bacterial and/or pesticidal agents.
There has been significant interest in developing transgenic plants that show increased resistance to a broad range of pathogens (Stuiver and Custers, 2001, Nature 411:865-8; Melchers and Stuiver, 2000, Curr Opin Plant Biol 3:147-52; Rommens and Kishore, 2000, Curr Opin Biotechnol 11:120-5; Mourgues et al. 1998, Trends Biotechnol 16:203-10). The interaction between Arabidopsis and the oomycete Peronospora parasitica (downy mildew) provides an attractive model system to identify molecular components of the host that are required for recognition of the fungal parasite (Parker et al. 1996 Plant Cell 8:2033-46). A number of genes whose mis-expression is associated with altered resistance to P. parasitica, as well as other pathogens, have been identified in Arabidopsis. Overexpression of the NPR1 gene confers resistance to infection by P. parasitica as well as the bacterial pathogen Pseudomonas syringae (Cao et al, 1998 Proc Natl Acad Sci USA 95:6531-6536). CPR6 is semi-dominant mutation implicated in multiple defense pathways (Clarke et al. 1998, Plant Cell 10:557-569). Lsd6 and Lsd7 are dominant mutations that confer heightened disease and result in the development of spontaneous necrotic lesions and elevated levels of salicylic acid (Weymann et al 1995 Plant Cell 7:2013-2022). A number of recessive mutations confer P. parasitica resistance, including ssi2, in the SSI2 gene encoding a stearoyl-ACP desaturase (Kachroo et al. 2001 Proc Natl Acad Sci USA 98:9448-9453), mpk4, in a MAP kinase gene (Petersen et al. 2000, Cell 103:1111-20), and pmr4 (Vogel and Somerville 2000 Proc Natl Acad Sci USA 97:1897-1902). The recessive mutations cpr5 and cpr1 also confer resistance to P. syringae and cause a dwarf phenotype (Bowling et al 1997 Plant Cell 9:1573-1584; Bowling et al, 1994 Plant Cell 6:1845-1857).
Activation tagging in plants refers to a method of generating random mutations by insertion of a heterologous nucleic acid construct comprising regulatory sequences (e.g., an enhancer) into a plant genome. The regulatory sequences can act to enhance transcription of one or more native plant genes; accordingly, activation tagging is a fruitful method for generating gain-of-function, generally dominant mutants (see, e.g., Hayashi et al., Science (1992) 258: 1350-1353; Weigel et al., Plant Physiology (2000) 122:1003-1013). The inserted construct provides a molecular tag for rapid identification of the native plant whose mis-expression causes the mutant phenotype. Activation tagging may also cause loss-of-function phenotypes. The insertion may result in disruption of a native plant gene, in which case the phenotype is generally recessive.
Activation tagging has been used in various species, including tobacco and Arabidopsis, to identify many different kinds of mutant phenotypes and the genes associated with these phenotypes (Wilson et al., Plant Cell (1996) 8:659-671, Schaffer et al., Cell (1998) 93: 1219-1229; Fridborg et al., Plant Cell (1999)11: 1019-1032; Kardailsky et al., Science (1999) 286:1962-1965); Christensen S et al., 9th International Conference on Arabidopsis Research. Univ. of Wisconsin-Madison, Jun. 24-28, 1998. Abstract 165). In one example, activation tagging was used to identify mutants with altered disease resistance (Weigel et al., supra).